Metal-metal bonded ammonia oxidation catalysts

ABSTRACT

Methods and catalysts for oxidizing ammonia to nitrogen are described. Specifically, diruthenium complexes that spontaneously catalyze this reaction are disclosed. Accordingly, the disclosed methods and catalysts can be used in various electrochemical cell-based energy storage and energy production applications that could form the basis for a potential nitrogen economy.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional Application No.62/924,761 filed on Oct. 23, 2019, which is incorporated by referenceherein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-SC0016442awarded by the US Department of Energy. The government has certainrights in the invention.

FIELD OF THE INVENTION

This disclosure is directed to diruthenium complexes and the use of suchcomplexes to catalyze the oxidation of ammonia.

BACKGROUND OF THE INVENTION

Ammonia (NH₃) is a colorless, pungent gas. In its liquid form, ammoniais an excellent source of hydrogen (containing twice as much hydrogen byvolume as liquid hydrogen). In addition, there are existing supplychains and regulations, which allow for widespread distribution. As aresult, ammonia is of interest as a fuel in fuel cells and otherapplications, where ammonia may be oxidized to nitrogen using an oxidantsuch as oxygen. However, the chemical interconversion between nitrogenand ammonia has remained a great challenge for chemists.

A functional “nitrogen economy” with widespread use of ammonia as a fuelrequires the development of two primary technologies: (1) the efficientsynthesis of ammonia from air, light, and water; and (2) the reversereaction—the efficient oxidation of ammonia to power fuel cells.

Major efforts in the field of inorganic chemistry have sought to addresseach of these challenges. It is well established that ammonia bindstightly to transition metals, making it difficult to activate. Numerousefforts to activate ammonia using transition metal complexes haveresulted in steady progress, particularly electrocatalytic approaches,which use an applied potential to improve reaction kinetics. However,the overpotential needed to drive such reactions (i.e., the amount ofadditional energy required beyond what is thermodynamically expected) istoo high to be practical using prior catalysts. Thus, reducing theoverpotential or identifying catalysts that do not require addedpotential is the key to enabling direct ammonia fuel cell technology.

Accordingly, there is need for the development of catalysts and methodsfor oxidizing ammonia to nitrogen without applying a substantialoverpotential. Such catalysts and methods could be used as the basis fora nitrogen economy using ammonia fuel cells to store and supply energy.

BRIEF SUMMARY

We disclose herein diruthenium complexes that spontaneously react withammonia to form nitrogen. These complexes also catalyze the oxidation ofammonia to nitrogen with a low overpotential. Accordingly, suchcompounds are a significant improvement as compared to previously knowncatalysts for the oxidation of ammonia and could form the basis of acost-effective and scalable ammonia fuel cell that could support apotential nitrogen economy.

In a first aspect, the disclosure encompasses a diruthenium complexhaving the chemical structure:

The central diruthenium is [Ru]₂ ^(n+), where n is 3-7; L, which may ormay not be present, is, if present, a non-competitively binding ligand;each E, which may be the same or different, is independently O, S, NH orNY, where Y is an alkyl or an aryl group; each X, which may be the sameor different, is a steric tuning group; and R, which may or may not bepresent, is (if present) an electronic tuning group.

The diruthenium complex is subject to the following provisos. If L isBF₄ and each E is O, then all four X are not Cl; if L is Cl and each Eis O, then all four X are not Cl or F; and if L is OMe, THF, orpyridine, and each E is O, then all four X are not Cl.

In some embodiments, L is trifluoromethanesulfonate (OTf), NH₃, BF₄, Cl,OCH₃ (OMe), tetrahydrofuran (TF) or pyridine.

In some embodiments, each E is NH or NY, where Y is methyl (Me) orphenyl (Ph).

In some embodiments, each E is O.

In some embodiments, each X is a halogen, a methyl (Me), or a —(CH)₄—,where the —(CH)₄— is attached at one end to the carbon atom attached toX, and attached at the other end to the carbon atom directly adjacent tothe carbon atom attached to X, forming a fused bicyclic aromatic(quinoline) core.

In some embodiments, each X is Cl, F, or Me.

In some embodiments, R is not present.

In other embodiments, R is present. In some such embodiments, R istrimethylsilyl (TMS) or bromine.

In some embodiments, L is OTf; each E is O; each X is Cl, F, or Me; andR is not present. In some such embodiments, each X is Cl. In other suchembodiments, each X is F. In yet other such embodiments, each X is Me.

In some embodiments, L is NH₃; each E is O; each X is Cl, F, or Me; andR is not present. In some such embodiments, each X is Cl. In other suchembodiments, each X is F. In yet other such embodiments, each X is Me.

In a second aspect, the disclosure encompasses a method of oxidizing NH₃to N₂. The method includes the step of contacting a catalyst comprisinga diruthenium complex with NH₃, which results in the oxidation of NH₃ toN₂.

In some embodiments, the NH₃ is spontaneously oxidized to N₂ withoutapplying an externally generated electrical potential.

In some embodiments, the diruthenium complex has the chemical structure:

The central diruthenium is [Ru]₂ ^(n+), where n is 3-7; L is anon-competitive ligand; each E, which may be the same or different, isindependently O, S, NH or NY, where Y is an alkyl or an aryl; each X,which may be the same or different, is a steric tuning group; and R,which may or may not be present, is (if present) an electronic tuninggroup.

In some embodiments, L is BF₄, Cl, OCH₃ (OMe), tetrahydrofuran (THF),pyridine, trifluoromethanesulfonate (OTf) or NH₃.

In some embodiments, each E is NH or NY, where Y is methyl (Me) orphenyl (Ph).

In some embodiments, each E is O.

In some embodiments, each X is a halogen, a methyl (Me) or a —(CH)₄—,where the —(CH)₄— is attached at one end to the carbon atom attached toX, and attached at the other end to the carbon atom directly adjacent tothe carbon atom attached to X, forming a fused bicyclic aromatic core.

In some embodiments, R is not present.

In other embodiments, R is present. In some such embodiments, R istrimethylsilyl (TMS) or bromine.

In some embodiments, L is OTf; each E is O; each X is Cl, F, or Me; andR is not present. In some such embodiments, each X is Cl. In other suchembodiments, each X is F. In yet other such embodiments, each X is Me.

In some embodiments, L is NH₃; each E is O; each X is Cl, F, or Me; andR is not present. In some such embodiments, each X is Cl. In other suchembodiments, each X is F. In yet other such embodiments, each X is Me.

In some embodiments, L is BF₄; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is Cl; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is Cl; each E is O; each X is F; and R is notpresent.

In some embodiments, L is OMe; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is THF; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is pyridine; each E is O; each X is Cl; and R isnot present.

In some embodiments, the catalyst and the NH₃ are in contact with ananode electrode at which the oxidation of NH₃ to N₂ occurs. In some suchembodiments, the catalyst is in a solution of an organic solvent that isin contact with the anode electrode.

In some embodiments, the oxidation of NH₃ to N₂ occurs with low or nooverpotential.

In some embodiments, the anode electrode is part of an electrochemicalcell that further includes a cathode electrode at which a reductionreaction occurs. The cathode electrode is in fluid, ionic, and/orelectrical communication with the anode electrode. In some suchembodiments, the electrochemical cell is an energy storage cell, a fuelcell, or an electrosynthetic cell. In some such embodiments, theelectrochemical cell is a direct ammonia fuel cell.

In some embodiments, the electrochemical cell includes a membrane orbarrier separating the anode electrode from the cathode electrode.

In some embodiments, the cathode electrode includes or is in contactwith an oxygen reduction catalyst.

In some embodiments, O₂ is simultaneously reduced to H₂O at the cathodeelectrode.

In a third aspect, the disclosure encompasses an electrochemicalhalf-cell that includes an anode electrode in contact with a catalystthat includes a diruthenium complex. The catalyst is capable ofspontaneously catalyzing the oxidation of NH₃ to N₂.

In some embodiments, the catalyst is in a solution of an organic solventthat is in contact with the anode electrode.

In some embodiments, the anode electrode is further in contact with NH₃.In some such embodiments, the oxidation of NH₃ to N₂ is occurring at theanode electrode.

In some embodiments, the diruthenium complex has the chemical structure:

The central diruthenium is [Ru]₂ ^(n+), where n is 3-7; L is anon-competitive ligand; each E, which may be the same or different, isindependently O, S, NH or NY, where Y is an alkyl or an aryl; each X,which may be the same or different, is a steric tuning group; and R,which may or may not be present, is (if present) an electronic tuninggroup.

In some embodiments, L is BF₄, Cl, OCH₃ (OMe), tetrahydrofuran (THF),pyridine, trifluoromethanesulfonate (OTf) or NH₃.

In some embodiments, each E is NH or NY, where Y is methyl (Me) orphenyl (Ph).

In some embodiments, each E is O.

In some embodiments, each X is Cl, F, a methyl (Me) or a —(CH)₄—, wherethe —(CH)₄— is attached at one end to the carbon atom attached to X, andattached at the other end to the carbon atom directly adjacent to thecarbon atom attached to X, forming a fused bicyclic aromatic core.

In some embodiments, R is not present.

In some embodiments, R is present. In some such embodiments, R istrimethylsilyl (TMS) or bromine.

In some embodiments, L is OTf; each E is O; each X is Cl, F, or Me; andR is not present. In some such embodiments, each X is Cl. In other suchembodiments, each X is F. In yet other such embodiments, each X is Me.

In some embodiments, L is NH₃; each E is O; each X is Cl, F, or Me; andR is not present. In some such embodiments, each X is Cl. In other suchembodiments, each X is F. In yet other such embodiments, each X is Me.

In some embodiments, L is BF₄; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is Cl; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is Cl; each E is O; each X is F; and R is notpresent.

In some embodiments, L is OMe; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is TH; each E is O; each X is Cl; and R is notpresent.

In some embodiments, L is pyridine; each E is O; each X is Cl; and R isnot present.

In some embodiments, the electrochemical half-cell is part of anelectrochemical cell, where the electrochemical half-cell is in fluid,ionic and/or electrical communication with an electrochemical half-cellthat includes a cathode electrode. In some such embodiments, theelectrochemical cell is an energy storage cell, a fuel cell, or anelectrosynthetic cell.

In some embodiments, the electrochemical cell includes a membrane orbarrier separating the anode electrode and the cathode electrode.

In some embodiments, the cathode electrode includes a catalyst capableof catalyzing the reduction of O₂ to H₂O.

In some embodiments, when the electrochemical cell is in operation, O₂is being reduced to H₂O at the cathode electrode. In some embodiments,when the electrochemical cell is in operation, NH₃ is being oxidized toN₂ at the anode electrode.

In some embodiments, the electrochemical cell is a direct ammonia fuelcell.

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be better understood and features, aspects andadvantages other than those set forth above will become apparent whenconsideration is given to the following detailed description. Suchdetailed description makes reference to the following drawings. Thepatent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1A shows the generic chemical structure of the dirutheniumcomplexes that can act as a catalyst for the spontaneous oxidation ofammonia. The metal complexes include a central [Ru]₂ ^(n+) (n is 3-7),four equatorial ligands that together with the central [Ru]₂ ^(n+) coreform a “paddlewheel” structure, and an axial non-competitive ligand (L).

FIG. 1B shows the chemical structure of ten exemplary equatorial ligandsthat could be used to form the diruthenium complexes. These exemplaryequatorial ligands are non-limiting, in that other structurally similarequatorial ligands could also be used to form other dirutheniumcomplexes.

FIG. 2A is a schematic drawing that illustrates the essential componentsof a nitrogen economy.

FIG. 2B compares the chemical structure of previously publishedtransition metal complexes with ammonia with an exemplary dirutheniumammonia complex of the present disclosure.

FIG. 3 illustrates the synthetic scheme, crystal structures, andelectronic absorption data associated with the reaction of dirutheniumcomplex 2 with ammonia.

FIGS. 4A, 4B, and 4C show cyclic voltammograms for diruthenium complex 2in CH₃CN (4A, red trace), diruthenium complex 3 in CH₃CN (4A, purpletrace), diruthenium complex 6 in CH₃CN (4A, yellow trace), controlledcurrent traces for the electrochemical oxidation of ammonia withdiruthenium complex 6 in CH₃CN (4B), and electronic absorption tracescollected during electrochemical oxidation of ammonia with dirutheniumcomplex 6 in CH₃CN. Electrochemical experiments were conducted using 100mM Bu₄NPF₆ as the supporting electrolyte and potentials were referencedto Fc^(0/+).

FIGS. 4D, 4E, and 4F show electronic absorption spectra illustrating thereduction of diruthenium complex 3 to diruthenium complex 6 with 100equivalents of ammonia (4D), the subsequent oxidation of dirutheniumcomplex 6 to diruthenium complex 3 with oxygen (4E), and the reductionof diruthenium complex 3 to diruthenium complex 6 with an additional 100equivalents of ammonia (4F). Time intervals between scans are 5 minutes.

FIG. 5A shows a proposed catalytic cycle for the electrochemicaloxidation of ammonia.

FIG. 5B illustrates the results of a DFT orbital calculation showing a3-center π interaction in the proposed Ru—Ru—NH₂ species.

FIG. 5C illustrates a potential transition state showing the formationof a N—N bond by reaction of the proposed Ru—Ru—NH₂ species with NH₃.

FIG. 6 illustrates new Ru₂ complexes with H-bond accepting, donating,and non-H-bonding ligands alongside the crystal structures of their X═Clanalogs.

FIG. 7 illustrates sterically demanding ligands designed to discouragedimerization of reduced Ru₂ catalysts.

FIG. 8 illustrates modulation of hydrogen bonding near the axialcoordination site.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and are herein described in detail. Thedescription of specific embodiments is not intended to limit theinvention to the particular forms disclosed, but on the contrary, isintended to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION I. In General

This invention is not limited to the particular methodology, protocols,materials, and reagents described, as these may vary. It is also to beunderstood that the terminology used in this disclosure is for thepurpose of describing particular embodiments only and is not intended tolimit the scope of the present invention, which will be limited only bythe language of the appended claims.

As used in this disclosure and in the appended claims, the singularforms “a”, “an”, and “the” include plural reference unless the contextclearly dictates otherwise. The terms “a” (or “an”), “one or more” and“at least one” can be used interchangeably. The terms “comprising”,“including”, and “having” can also be used interchangeably.

Unless defined otherwise, all technical and scientific terms used inthis disclosure, including element symbols, have the same meanings ascommonly understood by one of ordinary skill in the art. Chemicalcompound names that are commonly used and recognized in the art are usedinterchangeably with the equivalent IUPAC names.

All publications and patents specifically mentioned in this disclosureare incorporated by reference for all purposes.

II. The Invention

This disclosure is based on our discovery that diruthenium complexeswith the oxidation state [Ru₂]⁵⁺ can spontaneously oxidize ammonia tonitrogen. Notably, the disclosed diruthenium complexes catalyze theammonia oxidation half-reaction upon reoxidation from [Ru₂]⁴⁺ to [Ru₂]⁵⁺with the overpotential set by the [Ru₂]^(4+/5+) redox potential.Accordingly, the diruthenium complexes could be used in improved fuelcells for energy storage applications, such as in direct ammonia fuelcells.

Structure of the Diruthenium Complex Catalysts

The diruthenium complex catalysts have the general chemical structureshown in FIG. 1A.

As seen in FIG. 1A, the general structure includes a diruthenium core atthe center (Ru—Ru). The oxidation state of the diruthenium core mayvary. Specifically, the diruthenium may have a charge of +3 to +7 (i.e.,the diruthenium can be designated as [Ru]₂ ^(n+), where n=3-7).

As further seen in FIG. 1A, four equatorial ligands are arranged aroundthe diruthenium at 900 intervals, forming a square planar “paddlewheel”structure. The equatorial ligands are kinetically inert. The fourequatorial ligands may be structurally the same, or they may bestructurally different.

For each equatorial ligand, E is O, NH, N(aryl), N(alkyl) or S. Anon-limiting example of N(aryl) is N(phenyl), where the phenyl may besubstituted or unsubstituted. A non-limiting example of N(alkyl) isN(methyl).

For each equatorial ligand, X is a steric tuning group. Non-limitingexamples include halogens such as Br or Cl, alkyl groups such as methyl,or an aromatic ring that is installed in such a way as to form a fusedbicyclic aromatic core that may be substituted or unsubstituted. Aspecific example of such X group is —(CH)₄—, where the —(CH)₄— isattached at one end to the carbon atom attached to X, and attached atthe other end to the carbon atom directly adjacent to the carbon atomattached to X (i.e., a substituted quinoline).

Each equatorial ligand may also include R, an electronic tuning group.The presence of R is optional and not required. Non-limiting examplesfor R include halogens such as Br, trimethylsilyl (TMS), or a fusedaromatic ring as described in the previous paragraph.

Ten non-limiting examples of possible equatorial ligands are shown inFIG. 1B.

As seen in FIG. 1A, the general complex structure further may or may notinclude a non-competitive ligand, L, arranged in an axial positionrelative to the four equatorial ligands. Accordingly, the five ligandsform a square pyramidal geometry around one Ru atom of the central [Ru]₂^(n+) core, with the axial ligand L at the peak of the pyramid and thefour equatorial ligands at the corners of the square. Non-limitingexamples for L include NH₃, trifluoromethanesulfonate (OTf), BF₄, Cl,OCH₃ (OMe), tetrahydrofuran (THF), or pyridine.

The following examples are offered for illustrative purposes only andare not intended to limit the scope of the invention in any way. Indeed,various modifications of the invention in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and the following examples, falling within thescope of the appended claims.

III. EXAMPLES Example 1: Synthesis and Use of an Exemplary DirutheniumComplex to Spontaneously Catalyze the Oxidation of NH₃ to N₂

This example provides a “proof of principle” by demonstrating the use ofan exemplary diruthenium catalyst to oxidize ammonia to nitrogenspontaneously and with no overpotential.

The chemical interconversion between nitrogen and ammonia is one of themost difficult mechanistic challenges for chemists, but it is one thatholds the potential to radically transform the world's energy economy. Afunctional Nitrogen Economy (FIG. 2A) requires the development of twokey technologies: (1) ammonia synthesis from air, light, and water, andits reverse; (2) ammonia oxidation to power fuel cells.

Major efforts in the field of inorganic chemistry have sought to addresseach of these challenges. Ammonia has been known to bind to transitionmetal salts since the early 1800s. The resulting ammine complexes aregenerally very stable and the ammonia ligand is difficult to activate,as in the classic Werner-type complexes [Ru(NH₃)₆]^(n+) (n=2, 3).Highlights of recent efforts to activate ammonia using transition metalcomplexes include: the coordination of NH₃ to low-valent metal centersreported by Chirik and coworkers to spontaneously eliminate H₂, andelimination of N₂ from Ru—NH₃ complexes upon oxidation. In the lattercase, Hamann, Smith, and coworkers developed an electrocatalytic systemto split ammonia into N₂ and H₂ at an applied potential of 0.20 V vsFc^(0/+), an overpotential (I) of 1.47 V vs the thermodynamic standardreduction potential (N₂+6e⁻+6H₂O→2 NH₃+6OH⁻; E°=−1.27 V vs Fc^(0/+))(Habibzadeh, F.; Miller, S. L.; Hamann, T. W.; Smith, M. R. Homogeneouselectrocatalytic oxidation of ammonia to N₂ under mild conditions.Proceedings of the National Academy of Sciences 2019, 116, 2849-2853.)

Reducing the η is the key to making direct ammonia fuel cell technologypossible. In this example, we report a metal-metal bonded dirutheniumcomplex 2 that spontaneously forms nitrogen from ammonia without anyapplied potential. Moreover, we have found that the dirutheniumpaddlewheel complex 2 can be oxidized and reduced in batch cycles usingoxygen as the sacrificial oxidant, demonstrating the feasibility of 2 asa catalyst for the chemical reactions necessary for a direct ammoniafuel cell.

We have focused on the ability of d-orbital interactions in metal-metalbonded compounds to facilitate multi-electron reactions. Recently, wehave explored the chemistry of Ru₂ compounds supported by6-chloro-2-hydroxypyridinate (chp⁻) equatorial ligands, as shown in FIG.2B (rightmost structure). These ligands support a variety of compoundshaving the [Ru₂]⁵⁺ oxidation state that are air stable and deep purplein color. These compounds may also be chemically reduced the [Ru₂]⁴⁺state giving compounds that are generally air sensitive and brown incolor. In both the [Ru₂]⁴⁺ and [Ru₂]⁵⁺ oxidation states, the compoundsare paramagnetic with S=1 and S=3/2, respectively.

The new [Ru₂]⁵⁺ complex (Ru₂(chp)₄OTf), 2, was prepared by metathesis ofRu₂(chp)₄Cl with TlOTf. Addition of NH₃ to 2 at −25° C. yields 3([Ru₂(chp)₄NH₃]OTf), which has been crystallographically characterized(See FIG. 3). The crystal structure displays intermolecular hydrogenbonding interactions that can be tamed when 3 is exposed to 18-crown-6(18-c-6) at −25° C. to yield 4 ([Ru₂(chp)₄(NH₃)(18-c-6)]OTf). Thestructure of 4 shown in FIG. 3 reveals that all three hydrogens in theammine ligand are hydrogen-bonding to oxygen atoms in the 18-c-6. Atroom temperature, solutions of 3 are unstable and slowly produce anamber-colored species.

Upon addition of 18-c-6 to the mixture of 3 and its decompositionproducts, complex 7 ([Ru₂(chp)₄(NH₃)]2(18-c-6)) can be crystallized.This sandwich complex is structurally similar to 4, but instead displaysa total of six hydrogen bonds between the 18-c-6 and each ammine ligandin two diruthenium complexes. Complex 6 may also be synthesizedindependently from dimeric 5 ([Ru₂(chp)₄]₂), a dimeric complex formed byzinc reduction of 1 (Brown, T. R.; Dolinar, B. S.; Hillard, E. A.;Clérac, R.; Berry, J. F. Electronic Structure of Ru₂(II,II)Oxypyridinates: Synthetic, Structural, and Theoretical Insights intoAxial Ligand Binding. Inorg. Chem. 2015, 54, 8571-8589) and subsequentreaction with an aqueous or non-aqueous solution of ammonia. Addition of18-c-6 to 6 also leads to crystallization of 7.

Through deliberate synthesis of 6 and 7 we were able to confirm that thereaction of the [Ru₂]⁵⁺ complex with ammonia results in a one-electronreduction to a [Ru₂]⁴⁺ complex. Thus, ammonia must be oxidized, and wetherefore pursued characterization of the other products in thisreaction.

Stirring 2 with excess ammonia results in complete conversion to 6, asmonitored via electronic absorption spectroscopy (see FIG. 3, lowerleft). Additionally, NMR spectroscopic characterization of the materialrecovered from this reaction shows a distinct triplet assigned to NH₄ ⁺.

To test for the formation of N₂, we subjected 2 to excess ¹⁵NH₃ in anargon atmosphere and found ¹⁵N¹⁵N in the headspace. A 40% yield of¹⁵N¹⁵N was determined by mass spectrometry analysis of the reactionheadspace, on the basis of a presumed 6:1 molar ratio between 2 and N₂(Equation 1). This value should be considered to be a lower bound forthe true yield of N₂, which is difficult to analyze quantitatively.Remarkably, we have obtained a yield of 12% when using aqueous ¹⁵NH₃,demonstrating that water hinders the ammonia oxidation reaction but doesnot shut down the reaction entirely.

$\begin{matrix}\left. {\underset{2}{6\mspace{11mu}{{Ru}_{2}({chp})}_{4}{OTf}} + {14\mspace{11mu}{NH}_{3}}}\rightarrow{N_{2} + \underset{6}{6\mspace{11mu}{{Ru}_{2}({chp})}_{4}\left( {NH}_{3} \right)} + {6\mspace{11mu}{NH}_{4}{OTf}}} \right. & {{Equation}\mspace{14mu} 1}\end{matrix}$

Since 2 contains a [Ru₂]⁵⁺ core, a one-electron oxidized form of the[Ru₂]⁴⁺ core in 6, we explored the electrochemical features of 2 to seewhether an electrochemical cycle for ammonia oxidation could bedeveloped.

The cyclic voltammogram of 2 in CH₃CN (See FIG. 4A, red trace) shows areversible event assigned to the [Ru₂]^(4+/5+) redox couple atE_(1/2)=−128 mV vs Fc^(0/+), as well as a second reversible event atE_(1/2)=−347 mV assigned to the [Ru₂]^(4+/5+) redox couple for thesolvent complex Ru₂(chp)₄(NCCH₃), which is in equilibrium with 2. Theaddition of 0.8 equivalents of ammonia to this solution creates a newreversible feature at E_(1/2)=−255 mV while completely eliminating thefeature assigned to the [Ru₂]^(4+/5+) couple of 2 (See FIG. 4A, purpletrace). The new feature is assigned to the [Ru₂]^(4+/5+) redox couplefor the [Ru₂(chp)₄(NH₃)]⁺ cation in 3 (i.e., 6⁺), which is confirmed byits appearance in voltammograms collected for 6, the one-electronreduced analogue Ru₂(chp)₄(NH₃) (See FIG. 4A, yellow trace).

Constant current electrolysis of 6 was performed in a divided cell witha reticulated vitreous carbon (RVC) working electrode and a platinum rodcounter electrode in a solution of ferrocenium hexafluorophosphate(FcPF₆) as the sacrificial oxidant. Electronic absorption spectra weresimultaneously collected throughout each experiment.

With no excess added ammonia (minimal ammonia concentration of <5 mM,See FIG. 4B, brown trace), the applied electrode potential required tosustain 0.5 mA current begins at −245 mV, approximately the E_(1/2) of6, and starts to increase rapidly after 36 minutes (1.08 C chargepassed, 0.68 e⁻ equivalents vs [Ru₂]) before plateauing at ˜600 mV. Theabrupt shift to higher applied potentials marks a transition from[Ru₂]⁴⁺ oxidation to a different predominant electrode process,indicating depletion of 6 and formation of 6⁺. Electronic absorptiondata (See FIG. 4C, brown trace) are consistent with this interpretationand show a charge-dependent linear increase in characteristic Ru₂ ⁵absorbances at 530 nm and 675 nm that plateaus after the passage of onecharge equivalent (See FIG. 4C, 1 F/mol mark), indicative ofstoichiometric bulk oxidation from [Ru₂]⁴⁺ to [Ru₂]⁵⁺ species,respectively amber and purple in color. The introduction of excess ¹⁵NH₃to the solution after bulk oxidation yields the same color change frompurple to amber seen in the reaction of 2 with ammonia, indicative ofreduction back to a [Ru₂]⁴⁺ state. Additionally, ¹⁵N¹⁵N was detected inthe headspace by mass spectrometry, confirming that N₂ is formed fromoxidation of ammonia.

When a similar experiment is conducted in the presence of excess ammonia(235 mM, 1070 equiv. vs [Ru₂], See FIG. 4B, blue trace), 0.5 mA currentis sustained for over 2.5 hours at low potentials ranging from −284 to−121 mV, with no increase to potentials capable of direct AOR at the RVCelectrode (˜400 mV, See FIG. 4B, green trace). Electrolysis at the[Ru₂]^(4+/5+) redox couple thus continues to be the predominantelectrode process even after the passage of 4.4 C, well beyond thecharge required for stoichiometric one-electron oxidation of 6 (1.6 C).This implies that exogenous ammonia continually reduces 6⁺ formedelectrolytically over the course of the experiment, thereby regenerating6 and delaying the full conversion to [Ru₂]⁵⁺ products. Concomitantelectronic absorption data (See FIG. 4C, blue trace) confirm thishypothesis, showing a diminished net rate of [Ru₂]⁵⁺ formation comparedto bulk electrolysis performed at an identical current but lowerconcentration of ammonia. Modeling of these data indicatespseudo-first-order kinetics in [Ru₂]⁵⁺ for the rate-determining ammoniaoxidation step (k_(obs)=5.63×10⁻⁴ s⁻¹ at 235 mM NH₃). The sustainedpassage of superstoichiometric charge at potentials near the[Ru₂]^(4+/5+) redox couple therefore demonstrates 6⁺ as an effectiveredox mediator for promoting AOR electrocatalytically. This systemdisplays surprising longevity of electrocatalytic activity, aselectrolysis was performed for nearly 3.5 hours at potentials well underthose required to oxidize ammonia in the absence of 6.

The surprising accessibility of the [Ru₂]^(4+/5+) redox event forcomplex 6 prompted us to explore the re-oxidation of 6 with oxygen.Addition of 100 equivalents of NH₃ in acetonitrile to 2 results in theimmediate formation of 3 followed by slow transformation to 6 over 1.5hours with pseudo-first order kinetics, as observed by electronicabsorption spectra (FIGS. 4D-F) and a color change from purple to amber.After sparging the solution with N₂ to clear away excess NH₃, a streamof O₂ was bubbled through the solution, resulting in fast oxidation of 6back to the [Ru₂]⁵⁺ oxidation state as observed in the electronicabsorption spectra and a color change from brown back to purple. Another100 equivalents of ammonia were added, and reduction back to 6 wasobserved. This experiment demonstrates that our diruthenium complexescan perform the fundamental chemistry necessary for a direct ammoniafuel cell.

Efforts then turned to investigating the mechanism of the transformationproposed in Equation 1. Given ammonia's apparent dual role both as areductant and as a Brønsted base, preliminary experiments sought toprobe the latter function by examining the effect of added NH₄ ⁺ to thereduction of 2 with NH₃. The presence of ˜20 equivalents of NH₄PF₆(versus 2) slowed the reduction by a factor of six, which suggests amechanism with at least one deprotonation step prior to therate-determining step.

Thus, we investigated the intermediacy of a neutral amido complex,Ru₂(chp)₄NH₂. Such complex would possess a nitrogen-centered lone pairthat could participate in a three-centered π interaction with thediruthenium core. Preliminary electronic structure calculations suggestthat this interaction lifts the degeneracy of Ru—Ru π* orbitals anddestabilizes the Ru—Ru—N_(amido) π* orbital, which becomes the LUMO ofRu₂(chp)₄NH₂. This feature also confers electrophilic character to theamido ligand, thus providing an avenue to N—N bond formation throughnucleophilic attack by ammonia at this site (see FIGS. 5A-C).

In sum, these results demonstrate that the disclosed dirutheniumcomplexes can spontaneously catalyze the oxidation of ammonia tonitrogen. Because of the surprising and unexpected ability of thecomplexes to catalyze this reaction with low overpotential, as comparedto other metal complex-based catalysts, the disclosed catalysts andmethods have the potential to form the basis of a much-expanded andcommercially significant nitrogen economy.

Example 2: Further Considerations for Expanded New Catalyst Development

In this partially prophetic example, we discuss aspects of the chemicalstructures of the disclosed diruthenium complexes that could be tuned tooptimize their ability to catalyze the AOR.

Steric and Electronic Tuning

The reactivity of Ru₂(chp)₄(NH₃) is significantly distinct from that ofmononuclear Ru-ammine complexes, suggesting a potential feature in themetal-metal multiply bonded platform that can be leveraged to facilitatethe AOR. Thus, we are investigating a class of Ru₂ complexes thatdisplay a tunable primary and secondary ligand sphere.

An important feature of the disclosed Ru₂(ligand)₄ complexes is thatthey must be obtainable in their polar “4,0” isomer. In the case ofN,O-donor ligands like the chp, this means that one Ru atom is bound bythe four O atoms of the equatorial ligands while the other Ru atom isligated by the four ligand N atoms. Other isomers are possible, and anexploration of synthetic techniques is necessary in order to find outhow to specifically obtain only the desired 4,0 isomers of the catalystprecursors. We discuss here the new ligand architectures for which wehave already done this exploratory work and have obtained 4,0 complexes.

In our mechanistic work on the Ru₂(chp)₄(NH₃) catalyst, we haverecognized that the O atoms of the chp ligands display hydrogen bondaccepting character. In order to probe the importance of secondcoordination sphere hydrogen bonding interactions in the AOR, we havesought to prepare a series of Ru₂ catalysts that contain hydrogen bondaccepting groups (O atoms), hydrogen bond donating groups (NH groups),or non-hydrogen bonding groups at the periphery of the active site (seeFIG. 6). The new oxypyridinate, oxyquinolinate, and aminoquinolinatecomplexes shown in FIG. 6 have been examined in unpublished work, whilewe have published several reports detailing the fundamental aspects ofRu₂-anilinopyridinate (ap) complexes (Corcos, A. R.; Roy, M. D.;Killian, M. M.; Dillon, S.; Brunold, T. C.; Berry, J. F. ElectronicStructure of Anilinopyridinate-Supported Ru₂ ⁵⁺ Paddlewheel Compounds.Inorg. Chem. 2017, 56, 14662-14670; Corcos, A. R.; Berry, J. F.Anilinopyridinate-supported Ru₂ ^(x+) (x=5 or 6) paddlewheel complexeswith labile axial ligands. Dalton Trans. 2017, 46, 5532-5539; Corcos, A.R.; Berry, J. F. Capturing the missing [AgF₂]⁻ anion within anRu₂(III/III) dimeric dumbbell complex. Dalton Trans. 2016, 45,2386-2389).

We have found that NH₃ reacts with Ru₂(ap)₄(OTf) to form[Ru₂(ap)₄(NH₃)]OTf, as inferred by its spectroscopic similarity to theknown compound [Ru₂(ap)₄(NCCH₃)]BF₄. The facile synthesis ofanilinopyridinate ligands using substituted anilines has allowed thepreparation of a series of new diruthenium complexes with electronicallydiverse aryl rings. The ability to tune the electronic properties ofthis diruthenium system without influencing the steric environmentaffords a versatile platform for exploring how the redox potentials ofthe catalysts affect the catalytic AOR reaction.

Steric Prevention of Dimerization

One issue that has impeded our efforts towards isolating potential AORintermediates is the propensity of Ru₂(chp)₄ species to dimerize uponreduction to the [Ru₂]⁴⁺ oxidation state, as in the formation of[Ru₂(chp)₄]₂. We have synthesized new 4,0-Ru₂ complexes that stericallyprohibit dimerization through bulky substitution at the 3-position ofchp, such as those with Br₂chp and (Me₃Si)₂chp (FIG. 7, Brown, T. R.;Lange, J. P.; Mortimer, M. J.; Berry, J. F. New OxypyridinatePaddlewheel Ligands for Alkane-Soluble, Sterically-Protected Ru₂(II,III)and Ru₂(II,II) Complexes. Inorg. Chem. 2018, 57, 10331-10340). Thesynthesis of these ligands and corresponding Ru₂ complexes is difficultand inefficient, since selective substitution at the 3-position of chpis not electronically favorable.

We discovered that commercially available 2-hydroxyquinolinate (hq⁻) canbe used to prepare Ru₂(hq)₄Cl (FIG. 6) with reaction conditionsanalogous to those used for the synthesis of Ru₂(chp)₄Cl. Sincesubstitutions at the 3-position in 2-hydroxyquinoline with methyl,Me₃Si, and other groups are reported in the literature (González, R.;Ramos, M. T.; De la Cuesta, E.; Avendaño, C. Base-catalyzedElectrophilic Substitution in 2(1H)-Quinolinones. Heterocycles 1993, 36,315-322), we can readily explore the synthesis of Ru₂ complexessupported with these substituted ligands and also study their reactivitywith ammonia.

The crystal structure of Ru₂(chp)₄(N₂H₃Ph) features an intramolecularhydrogen bonding interaction that could potentially play an importantrole in stabilizing intermediates or transition states involved in theAOR. In order to study systematically the effect of hydrogen bondingnear the axial coordination site, we can prepare chp and hq analogs thatare expected to display distinct hydrogen bonding character. Ligandmodifications are shown in FIG. 8 (all three ligands are commerciallyavailable, as are their hq analogs). In preliminary work, we haveprepared the first examples of diruthenium hq and aminoquinolinate (aq)complexes.

We have also explored the chemistry of anilinopyridinate (ap) complexes,which do not offer any hydrogen bonding donors or acceptors in thesecond coordination sphere and are more electron-rich than the otherligands just described. The ap ligands are highly tunable based onelectron donating/withdrawing properties of the anilines from which theyare derived. For all ligand classes, we envision complexes with axialNH₃ ligands. It is anticipated that, in some cases, spontaneousreduction to the [Ru₂]⁴⁺ state will occur, whereas some complexes (i.e.,those with more electron rich equatorial ligands) will yield stable Ru₂⁵⁺—NH₃ complexes. Once stable NH₃ complexes are accessed, theirelectrochemical properties will be probed in the absence and presence ofexcess NH₃ to screen for catalytic AOR reactivity.

In sum, these examples illustrate possible structural modifications fortuning the electronic and steric properties of the disclosed dirutheniumcomplexes to optimize their catalytic AOR activity.

The invention is not limited to the embodiments set forth in thisdisclosure for illustration but includes everything that is within thescope of the appended claims.

We claim:
 1. A diruthenium complex having the chemical structure:

wherein: the central diruthenium is [Ru]₂ ^(n+), where n is 3-7; L isNH₃; each E is O; each X is Cl, F, or Me; and R is not present.
 2. Thediruthenium complex of claim 1, wherein each X is Cl.